The present application is based on Japanese priority application No.2000-205282 filed on Jul. 6, 2000, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to semiconductor devices and more particularly to a photodetector having an integral optical waveguide.
2. Description of the Related Art
With widespread use of information technology in human society, there is a demand for optical-fiber telecommunication systems that are capable of handling sharply increasing traffic of information.
In order to meet for this demand, there is a proposal to increase the transmission rate of the optical signals through the optical fibers from a conventional rate of 10 Gbps to a faster rate of 40 Gbps or more. When this approach is adopted, it is necessary to increase the response speed of the photodetector used in such an optical-fiber telecommunication system for detecting the optical signals.
Conventionally, PIN photodiodes have been used successfully in optical-fiber telecommunication systems as a high-speed photodetector. A PIN photodiode achieves a high response speed by providing a thin undoped semiconductor layer in a p-n junction such that the capacitance of the p-n junction is reduced.
Further, there is a proposal to increase the response speed and photosensitivity of a PIN photodiode further, by providing an integral optical waveguide adjacent to the PIN photodiode on a common semiconductor substrate.
FIG. 1 shows the construction of a conventional PIN photodetector 10 having such an integral optical waveguide.
Referring to FIG. 1, the PIN photodetector 10 is constructed on a compound semiconductor substrate 11 typically of InP and includes a PIN photodiode 12, wherein the PIN photodiode 12 includes therein an optical absorption layer 12A formed on the compound semiconductor substrate 11, and an optical waveguide 13 is provided on the compound semiconductor substrate 11 adjacent to the PIN photodiode 12 in optical coupling therewith.
The optical waveguide 13 includes an optical waveguide layer 13A having a first end surface to which an optical beam from an external optical waveguide, such as an optical fiber 14, comes in and a second end adjacent to the PIN photodiode 12 for injecting the optical beam guided through the optical waveguide layer 13A into the PIN photodiode 12. Further, there is provided a cladding layer 13B on the optical waveguide layer 13A for confining the optical beam in the optical waveguide layer 13A.
Thus, the optical waveguide layer 13A is provided in optical coupling with the optical absorption layer 12A of the PIN photodiode 12, and there occurs an efficient injection of the optical beam guided through the optical waveguide layer 13A into the optical absorption layer 12A of the PIN photodiode 12. The construction of FIG. 1 is effective for reducing the thickness of the optical absorption layer 12A below about 1 xcexcm without causing degradation of optical coupling between the optical waveguide layer 13A and the optical absorption layer 12A. As a result of decreased thickness of the optical absorption layer 12A, the PIN photodiode 12 shows a high response speed.
In such a PIN photodiode, it is possible to improve the response speed by providing the undoped optical absorption layer 12A between a p-type layer and an n-type layer constituting a PN junction so as to reduce the junction capacitance. On the other hand, such a construction has to be designed such that the optical absorption layer 12A has a sufficiently small thickness so as to avoid increase of the transit time of the optically excited carriers across the optical absorption layer 12A and associated degradation of the response speed of the PIN photodiode 12. From this viewpoint, it is desirable to reduce the thickness of the optical absorption layer 12A as much as possible.
On the other hand, there arises a problem, in such a conventional photodetector 10 having the optical waveguide 13 integrally to the PIN photodiode 12 and injection of optical signal occurs from the external optical fiber 14 into the PIN photodiode 12 via the optical waveguide 13, in that a large optical loss may occur at the first end surface of the optical waveguide 13 to which an optical signal in the optical fiber 14 is injected. It should be noted that the optical beam transmitted through the optical fiber 14 has a beam diameter of about 7.0 xcexcm in terms of full-height width, while the optical waveguide layer 13A in the optical waveguide 13 has a thickness of 1 xcexcm or less, which is substantially identical with the thickness of the optical absorption layer 12A in the PIN photodiode 12. When the thickness of the optical absorption layer 12A is increased for avoiding this problem, there arises a problem of degraded response speed of the photodiode 12.
As explained previously, it is desirable to suppress the thickness of the optical absorption layer 12A to be 1 xcexcm or less in the PIN photodiode 12 for improving the response speed. Further, it is preferable to reduce the longitudinal length of the photodiode 12A as much as possible so that the parasitic capacitance of the p-n junction is reduced. On the other hand, such a PIN photodiode having a short longitudinal length raises a problem in that the optical beam, entered into the PIN photodiode 12 at an incident end surface with offset from the optical absorption layer 12A, tends to exit from the opposite end surface before the optical beam is effectively confined into the optical absorption layer 12A by the optical confinement action.
In order to avoid these problems, there is a proposal to provide a spherical lens at an end of the optical fiber 14 such that the optical beam in the optical fiber 14 is injected efficiently into the thin optical absorption layer 12A of the PIN photodiode 12.
However, such a construction that uses a lens in the photodetector 10 is difficult to produce because of the stringent precision required for the optical system including the lens. As a result, such a construction inevitably increases the cost of the photodetector.
Accordingly, it is a general object of the present invention to provide a novel and useful photodetector wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a high-speed photodetector capable of minimizing optical loss with regard to an incoming optical beam having a large beam diameter.
Another object of the present invention is to provide a photodetector, comprising:
a substrate having a principal surface;
a photodetection part provided on a part of said principal surface of said substrate, said photodetection part comprising: an optical absorption layer of a semiconductor material extending parallel to said principal surface, said optical absorption layer causing excitation of carriers therein in response to an optical radiation supplied thereto; and
an optical waveguide provided on said principal surface of said substrate, said optical waveguide guiding an optical beam in a direction parallel to said principal surface from a first end surface to a second end surface adjacent to said photodetection part, such that said optical beam guided through said optical waveguide is injected into said optical absorption layer of said photodetection part, said optical waveguide comprising: a first tapered optical waveguide layer extending from said first end surface to said second end surface of said optical waveguide, said first tapered optical waveguide layer decreasing a thickness thereof continuously from said first end surface to said second end surface; a second tapered optical waveguide layer provided on said first tapered optical waveguide layer with a separation therefrom, said second tapered optical waveguide layer decreasing a thickness thereof continuously from said first end surface to said second end surface; and an intermediate layer having a refractive index smaller than a refractive index of any of said first and second tapered optical waveguide layers, said intermediate layer being interposed between said first and second tapered optical waveguide layers with a substantially uniform thickness.
According to the present invention, the effective refractive index of the optical waveguide, formed by the first and second tapered optical waveguide layers, increases sharply from the first end surface to the second end surface and the effect of optical confinement is enhanced sharply from the first end surface to the second end surface. As a result, a large-diameter optical beam incident to the first end surface of the optical waveguide is effectively confined into the first and second tapered optical waveguide layers as the optical beam travels from the foregoing first end surface to the second end surface and is injected into the optical absorption layer of the photodetection part effectively and efficiently. By using the two tapered optical waveguide layers, it becomes possible to increase the refractive index change between the first and second end surfaces as compared with the case of using a single tapered optical waveguide layer. Thus, the photodetector of the present invention provides a high optical coupling not achievable when a single tapered optical waveguide layer is used, while such a high efficiency of optical confinement enables use of a short longitudinal length of the photodiode constituting the photodetection part, and the response speed of the photodetector is improved.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.